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LANSING — Everybody knows water flows, but not many people know that the sediment below it does too. That’s why harbors need dredging, or excavating the gradually accumulated material on the bottom and transporting it elsewhere.

U.S. Army Corps of Engineers Detroit District planned eight dredging projects in Michigan and Wisconsin for 2014 at a cost of $13.2 million. But Congress recently allocated an additional $17.8 million. That allows the district to include eight new projects and increase funding for four of the original projects.

In all, Congress allocated an additional $46.5 million in dredging funds for the entire Great Lakes.

The number of federal dredging projects fluctuates yearly depending on funding, said Jim Tapp, chief of the Corps’ Technical Services Branch operations office. “There are always additional needs beyond those projects,” he said.

Lynn Rose, a Detroit District public affairs officer, said a number of sites, including the Detroit River, Green Bay Harbor, St. Marys River, Grand Haven Harbor, Holland Harbor and the Milwaukee River, that have high-value commercial traffic, are on the funding list every year. Other harbors may not have that opportunity.

The most recent contract was awarded to King Company of Holland, to remove extra sediment from Grand Haven and Holland harbors.

The two projects will be completed by late May at a cost of $657,500. Sediment dredged from the two harbors will be put along the nearby shoreline.

“The test result dictates where we will put the material,” Tapp said. Outer harbors are primarily made up of sand, which moves up and down the beaches and usually blocks off the entrance to the harbor. That kind of dredged material is clean and usually put on the beach.

The city manager of Grand Haven, Pat McGinnis, confirmed that the sandy sediment in Grand Haven Harbor was washed in from Lake Michigan. Putting the dredged material near shore is just to “put it back where it is from. There is no environmental concern,” McGinnis said.

The sandy material can also help prevent nearby beach erosion, according to the Corps’ report.

By comparison, inner harbors contain more material washed down from upland areas and the drainage basin. Such material “is more likely to be contaminated by the runoffs from farms and city street systems,” which might include petroleum and animal waste, McGinnis said.

If dredged material is contaminated, Tapp said, it will be transported to a controlled place, usually a confined disposal facility, to make sure it does not harm the environment.

McGinnis said inner harbor sediment in Grand Haven is mixed with organic substances to create topsoil for use in landscaping.

Commercial harbors, which require at least 21 feet of depth, are important for national and regional economies, McGinnis said. That’s where construction materials, like gravel and sand, are shipped in.

If the harbor isn’t dredged, “then we couldn’t get those materials,” McGinnis said. “We couldn’t build the roads. We couldn’t run our power plant. We couldn’t salt our roads.”

The key for maintenance dredging, according to McGinnis, is consistency. “When you let up and you allow these materials to stockpile, it’s really hard to catch up.”

The federal Harbor Maintenance Trust Fund was set up with taxes from shipping and boating industries to support maintenance dredging activities.

With dredging, “you’re never done because Mother Nature continues to bring these sediments back,” McGinnis said.

Detailed dredging information about the harbors in the Great Lakes Basin, including project costs, maps and the date when a harbor was last dredged can be found at http://www.lre.usace.army.mil/Missions/Operations.